This invention relates to transformers and more particularly to transformers having a dry-type construction with solid insulation.
A transformer with a dry-type construction includes at least one coil mounted to a core so as to form a core/coil assembly. The core is ferromagnetic and is often comprised of a stack of metal plates or laminations composed of grain-oriented silicon steel. The core/coil assembly is encapsulated in a solid insulating material to insulate and seal the core/coil assembly from the outside environment.
The solid insulating material that is used to encapsulate the core/coil assembly of a dry-type transformer is typically a thermoset polymer, which is a polymer material that cures, through the addition of energy, to a stronger form. The energy may be in the form of heat (generally above 200 degrees Celsius), through a chemical reaction, or irradiation. A thermoset resin is usually liquid or malleable prior to curing, which permits the resin to be molded. When a thermoset resin cures, molecules in the resin cross-link, which causes the resin to harden. After curing, a thermoset resin cannot be remelted or remolded, without destroying its original characteristics. Thermoset resins include epoxies, melamines, phenolics and ureas.
When a thermoset resin cures, the resin typically shrinks. Because the resin surrounds the core/coil assembly, the shrinking thermoset resin exerts high mechanical stresses and strains on the core of the transformer. These stresses and strains distort the oriented grains of the core and increase resistance to the magnetic flux flow in the laminations. This distortion and increased resistance results in higher core loss which causes the sensitivity of the transformer to decrease and diminishes the accuracy of the transformer. In addition, when the thermoset resin shrinks around edges and protrusions, cracks may form in the thermoset resin. The cracks may grow over time and compromise the insulating properties of the thermoset resin. As a result, partial discharges may occur. A partial discharge is an electrical spark that bridges the thermoset resin between portions of the core/coil assembly. A partial discharge doesn't necessarily occur at the core/coil assembly, it can occur anywhere the electric field strength exceeds the breakdown strength of the thermoset resin. Partial discharges contribute to the deterioration of the thermoset resin, which shortens the useful life of the transformer.
One approach for protecting the core of a transformer and preventing partial discharges has been disclosed in U.S. patent application Ser. No. 11/518,682, filed on Sep. 11, 2006, entitled “DRY-TYPE TRANSFORMER WITH SHIELDED CORE/COIL ASSEMBLY AND METHOD OF MANUFACTURING THE SAME”, which is assigned to the assignee of the present invention, ABB Technology AG, and which is incorporated herein by reference. In the '682 patent application, a core and coil assembly of a transformer are disposed inside a protective polymer case having an exterior surface that is at least partially covered with a conductive coating. The present invention is directed toward such a protective polymer case having an improved construction.